Photosensitive radiant-energy transducers



Nov. 26, 1963 w. o. REED 3,112,404

PHOTOSENSITIVE} RADIANT-ENERGY TRANSDUCERS Filed June 1'7, 1953 l3 l6 l3Photofignductive Element Photoconductlve Element ElectroluminescentElectroluminescent Element Element Voltage Voltage Fl G, 1 Source SourceFIG. 4

Electroluminescent Element 22 l3 l4 l5 l6 *5 j 1 O. .I I 5 4 Y o j :5 3.9 j 2 j A; E I (a I I E t I00 zoo 300 400 v, S Voltage acrossElectroluminescent conducflw/ |9 Layer Element 2 Voltage Source a: 400 QU 3300 2oo 2 I00 cc 0 l 2 3 4 5 Relative Intensity of Incident RadiationWILLIAM O. REED Fl G. 3 INVENTOR.

H'IS ATTORNEY.

United States Patent 3,112,404 PHOTGSENSITWE RADIANT-ENERGY TRANSDUQERSWilliam 0. Reed, Chicago, Ill, assignor to The Rauland Corporation, acorporation of Illinois Filed June 17, 1953, Ser. No. 362,195 2 Claims.(Q1. 250-213) This invention relates to radiant-energy transducers, andmore particularly, to such transducers comprising at least oneelectroluminescent element. For purposes of this specification,electroluminescence may be defined as the characteristic of emittingvisible or invisible light radiations in response to the application ofa suitable electric field.

Known radiant-energy transducers use the phenomenon of photoemission orcathodoluminescence to efiect the reproduction in observable form ofradiations representing an image. For example, image converters whichdepend upon the phenomenon of photoemission to translate light energy ofone wavelength into light energy at a diflerent wavelength, preferablyin the visible light spectrum, are well known. In general, devices ofthis type comprise photoemissive and fluorescent elements arrangedwithin an evacuated glass envelope, and while not exceptionally bulky orcumbersome, are not as compact as might be desired. Moreover, in thefabrication of such image converters, extreme care must be taken toavoid contamination of the fluorescent screen While activating thephotoemissive surface, and other costly and time-consuming operationsare involved in the manufacturing process.

In the field of projection television, one of the major limitations hasalways resided in the loss of picture brightness accompanying imagemagnificaiton by optical means. Electronic image intensification throughthe use of specially constructed image converters, while obtainable, hasnot been commercially feasible because of the inordinately high cost ofprojection television systems employing such devices as compared withthat of the structurally less complex and more compact arrangements fordirect-view image reproduction.

It is a primary object of this invention to provide an improvedradiant-energy transducer comprising at least one electroluminescentelement.

It is a further object of the invention to provide a new and improvedradiant-energy transducer for translating light radiations from onewavelength to a different wavelength.

It is another object of this invention to provide a novel radiant-energytransducer for intensifying or amplifying visible light radiations.

A further object of this invention is to provide a new and improvedimage intensifier well adapted for use in projection television systemsto increase the brightness of the reproduced image.

Yet another object of the invention is to provide a new and improvedradiant-energy transducer of simple and compact construction, which iswell adapted to economical fabrication on a mass production basis.

A new and improved radiant-energy transducer constructed in accordancewith the invention comprises a pair of electrodes with interposedcontiguous layers of radiant-energy-sensitive and electroluminescentmaterials respectively, in registration with the electrodes toconstitute a compact laminar structure. The radiant-energysensitivematerial has an impedance characteristic per unit area which is variablein response to incident radiations, while the electroluminescent elementemits light radiations in response to an applied electrical potential.One of the electrodes is at least substantially transparent to incidentradiations, and one of the electrodes is at least ice substantiallytransparent to the light radiations originating at theelectroluminescent layer. Means are also provided for establishing anelectrical potential difference between the electrodes to cause theelectroluminescent element to emit light in response to the variationsin the voltage applied to the electroluminescent element resulting fromvariations in the electrical impedance of the radiant-energy-sensitiveelement; the invention contemplates that the applied potential comprisesa combination of alternating and unidirectional voltages. The transducerof the invention may respond either to electromagnetic or particleradiation.

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The invention,together with further objects and advantages thereof, may best beunderstood, however, by reference to the following description taken inconnection with the accompanying drawings, in the several figures ofwhich like reference numerals indicate like elements, and in which:

FIGURE 1 is a cross-sectional view, partly schematic, of a preferredembodiment of the present invention,

FIGURE 2 and 3 are graphical representations of certain operatingcharacteristics of the embodiment of FIG- URE 1,

FIGURE 4 is a cross-sectional view, partly schematic, of anotherembodiment of the present invention, and

FIGURE 5 is a cross-sectional view, partly schematic, of a thirdembodiment of the invention.

A preferred embodiment of the invention, as illus trated in FIGURE 1,comprises a layer 14 of radiantenergy-sensitive or photosensitivematerial afiixed to a first electrode 13, and a layer 15 ofelectroluminescent material afilxed to a second electrode 16 anddisposed between the latter electrode and the side of photosensitivelayer 14 opposite electrode 13. For certain applications it may beadvantageous to provide a suitable binder, such as sodium or potassiumsilicate, between layers 14 and 15. From another viewpoint, electrodes13 and 16 oom prise the plates of a condenser the dielectric of which iscomposed of juxtaposed photosensitive and electroluminescent layers 14and 15. A source 2% of substantially constant alternating voltage isconnected to the electrodes 13 and 16 by means of wire conductors 19 orthe like.

For purposes of illustration there are also depicted an image it and anoptical lens system 11 The image 10 may be any object which is capableof emitting radiations or reflecting radiations representative of it.The optical lens system may be of any suitable construction, and, forconvenience, is here schematically represented as a single biconcavelens 11. Specifically, image 10 may constitute the image formed at thefluorescent screen of a projection-type cathode-ray tubeimage-reproducer of a television receiver, while lens system 11 maycomprise a Schmidt optical system or other image-magnifying lens systemassociated with the image reproducer.

The two electrodes 13 and 16 are composed of a material which iselectrically conductive and, in the preferred embodiment, electrode 13is at least substantially transparent to the incident light radiationsfrom image 10, while electrode 16 is at least substantially transparentto the light radiations emitted by the electroluminescent layer 15. Forexample, electrodes 13 and 16 may each be composed of a plate ofinsulating glass upon the inner surface of which is provided anelectrically conducting coating of tin oXide or the like; Corning E-C orelectrical conducting glass has been found satisfactory. The conductivecoating may be of the order of .00002 inch in thickness with a resultingtransparency of about with respect to the incident or the emitted lightradiations.

The radiant-energy-sensitive element or photosensitive layer 14 ispreferably formed of photoconductive mate- .9 rial, as for example leadsulphide or selenium in a transducer of the image intensifier typeintended for use at wavelengths within the visible portion of thespectrum, or thallous sulphide, lead telluride, or lead selenide in animage converter for translating infra-red light radiations to visibleimages. A typical composition of the electroluminescent material may beapproximately 80% zinc sulphide and Zinc selenide with copper as anactivator, although other electroluminescent materials such as siliconcarbide may be employed. Of course, the wavelength-responsecharacteristic of the transducer is determined largely by thecompositions of the photosensitive and electroluminescent layers;consequently the choice of materials is dependent on the application forwhich the transducer is intended. For convenience, the invention ishereinafter explained by reference to its image intensifier or lightamplifier embodiments, although it is to be clearly understood that suchembodiments may be modified to constitute image converters by merelyaltering the composition of the photosensitive material.

While the thicknesses of the photosensitive and electroluminescentlayers is partly dependent on the compositions employed and on thedesired operating characteristics, the photoconductive layer isgenerally thinner than the electroluminescent layer; in one embodiment,the photoconductive layer may be about microns thick, while thethickness of the electroluminescent layer may be of the order of 100microns, although thicknesses of much greater magnitude may be employed.

The operating characteristics of the transducer are materially afiectedby the frequency of the alternating voltage applied between electrodes13 and 16 from source 20. In general, for a given alternating voltage,light output is increased with an increase in the frequency. Thus,although the transducer is operative with 60-cycle alternating voltagesderived directly from the public utility power lines, it is preferred toemploy higher frequencies of the order of 2000 cps. or higher, in orderto achieve increased brightness; to this end, voltage source 2i) maycomprise a voltageand frequencystabilized audio-frequency oscillator.Moreover, with presently known materials, useful electroluminescence isonly achieved with applied alternating voltages exceeding apredetermined threshold voltage which is a function of the material;with the copper-activated zinc sulphide-zinc selenide described, thisthreshold voltage is about 200 volts R.M.S. If desired, a direct-voltagebias from any suitable source, as for example a battery or otherdirect-voltage power supply such as the rectified voltage supply of aprojection television receiver (not shown), may be superimposed ineither polarity on the alternating voltage applied between electrodes 13and 16.

The fabrication of the radiant-energy amplifier of the preferredembodiment may be accomplished in the following manner. Thephotosensitive or photoconductive element may be composed of leadsulphide which may be produced from natural sources, such as galenacrystals, or prepared synthetically. Commercial grades of galena ofchemical purity suificient for use in this device are readily available.The galena crystals are crushed into a fine powder and pressed through asuitable screen to provide minute crystals of uniform size. Thepulverized crystals may be sublimated to the desired thickness (about0.001 inch) on the surface of a section of electrically conducting glassto form the photoconductive layer 14 upon one side of electrode 13.During the process of sublimation it has been found advantageous tointroduce moist oxygen in order to aid materially in photosensitizingthe lead sulphide, although the reasons for the reaction of the moistoxygen with the lead sulphide to produce more sensitivity in thephotoconductive characteristic of this chemical compound are not fullyunderstood.

The electroluminescent element, comprised of a suitable composition ofzinc sulphide and zinc selenide with copper as an activator aspreviously discussed, may be suspended in a solution of ethylenedichloride and polyethyl methacrylate; if desired, a small amount ofbarium titanate or other high dielectric constant material may be addedfor the purpose of increasing the dark admittance of theelectroluminescent layer. The mixture is allowed to dry and the residueis crushed to a fine powder. The powder is pressed between two heatedsteel platens with accurately ground surfaces to form a filmapproximately microns thick. This film is then squeegeed against aconducting glass surface covered with a viscous grade of silicone oil.After the application of this sheet of electroluminescent material uponthe surface of the conductive glass, the excess oil is removed bycareful scraping so that there is intimate contact between the film andthe glass. The two glass sheets are then mounted together in such amanner that the coatings which have been placed on their surfaces are inregistration and in intimate contact with each other.

Much or" the theory involved in the function and nature ofelectroluminescence is not completely understood; however, it has beenexperimentally determined that if an alternating potential or arecurrent pulse signal of amplitude above a predetermined threshold isimpressed upon an electroluminescent material, the material emits lightradiation in proportion to the R.M.S. value of the alternating voltage.In short, the electroluminescent layer emits light radiation per unitarea with an intensity directly, although not necessarily linearly,proportional to the magnitude of the applied energizing signal. For thezinc sulphide-zinc selenide material described, the characteristic ofbrightness in foot lamberts emitted by the electroluminescent layer as afunction of the R.M.S. value of the impressed alternating potential isshown in FIGURE 2. It has been experimentally determined that thebrightness of films containing definite amounts of electroluminescentphosphor depends critically on the electric field strength, but onlyvery slightly on the amount of phosphors, provided there is at leastabout 6 milligrams of phosphor per square centimeter. Experiments haveindicated that within a temperature range from 100 C. to +50 C. thebrightness response is substantially independent of temperaturevariations.

FIGURE 3 shows the interdependence of the voltage components appliedrespectively to the photoconductive and electroluminescent layers as afunction of the intensity in foot lamberts of the incident light. Curve1 of FIGURE 3 represents a constant applied alternating voltage betweenelectrodes 13 and 16 of 400 volts R.M.S. Curve 2 of FIGURE 3 illustratesthe variation of that portion of the applied voltage which is impressedacross the electroluminescent layer as the intensity of incident lightis increased. Curve 3 represents the alternating voltage componentapplied across the photosensitive layer as a function of the intensityof the incident light. As shown by these characteristics, as therelative intensity of the incident light increases there is acorresponding decrease in the potential applied across thephotosensitive layer. There is also a correlative increase in thepotential applied across the electroluminescent element. For anycondition of incident light intensity, the sum of the voltage componentsacross the respective layers 14 and 15 (curves 2 and 3) corresponds tothe constant applied voltage of curve 1.

It is known that the dielectric constant of a photoconductive materialvaries in proportion to the intensity of incident light. Morespecifically as the incident light increases in intensity, thespecific-inductive-capacity or dielectric constant of thephotoconductive layer also increases. Furthermore, with an increase inincident light the resistance of the photoconduetive layer decreases topermit a greater flow of conductive current. Consequently, the twolayers constitute a capacitive voltage divider, with the voltagedivision ratio varying as a function of the incident light intensity. Asthe intensity of the incident light increases, the dielectric constantof the photoconductive material, and hence the voltage impressed acrossthe electroluminescent element, also increases. When the energizingsignal across the electroluminescent layer exceeds a certain thresholdvalue, an emission of visible light radiation from theelectroluminescent material ensues. In this Way the incident radiation,representative of image 10, which forms a charged area upon thephotosensitive layer 14, causes an image to be reproduced on theelectroluminescent layer 15. The variable response characteristics ofthe photosensitive and electroluminescent elements permit a reproductionof the image in gradations of brightness corresponding to the intensityof the incident radiations, and hence, corresponding to the half-tonesor shade values of the image.

The radiant-energy transducer of FIGURE 1 is considerably more compactthan hitherto known image intensifiers and image converters and may heconstructed and operated without being enclosed in an evacuatedenvelope; thus many of the difliculties encountered in the manufactureof previously known devices, such as the precautions againstcontamination of the fluorescent screen during activation of thephotoemissive cathode, are eliminated. Moreover, the transducer ofFIGURE 1 is readily adaptable to use in projection television systems toprovide increased brightness of the reproduced image, thus overcomingone of the most severe limitations heretofore encountered in suchsystems.

A further embodiment of this invention may be considered in conjunctionwith FIGURE 1 in which the voltage source 20 comprises a direct currentsource, such as an A.C. rectifier or battery, of about 400 volts D.C. Inthis embodiment image consists of a source of rapidly varyingillumination, such as the output of a motion picture projector, theviewing screen of a television picture tube, or a pulsed light source.In this case the transducer operates in a manner similar to thatdiscussed in the previous embodiment except that the change in impedancecharacteristic of the photosensitive layer causes the production of apulsed signal which although derived from a constant direct currentsource, has an alternating potential component. The electroluminescentlayer has of its very nature a leakage resistance which permits a decayin voltage across the photosensitive layer so that an effective constantpotential cannot remain across the photoconductive layer. In this mannera constant source of D.C. potential is effectively substituted for thealternating potential source which has proved to be necessary in otherembodiments of this invention.

An additional embodiment of this invention is shown in FIGURE 4. Thelaminar structure of FIGURE 4 and the general theory of operation ofthis structure are largely the same as discussed in connection with theembodiment of FIGURE 1. However, in this figure the incident light fromobject 10 is projected onto photoconductive layer 14 through electrode16 and electroluminescent layer 15. For such operation, electrode 16must be at least substantially transparent both to the incident lightradiation from object 10 and to the light radiations originating atelectroluminescent element electrode 13 need not be transparent, and ifdesired, may be formed as a highly polished metal plate or metal-coatedconductive glass plate to constitute a reflector immediately behindphotoconductive layer 14 so that a large portion of the light emittedfrom the electroluminescent layer may be utilized in conjunction withthe incident radiation to vary the efiective reactance and conductivityof the photosensitive element. This results in a regeneration of lightenergy which can prove to be very effective in the amplification of weakincident light radiation.

It may prove desirable in some applications to provide a means forcontrolling the amount of regeneration and for this purpose a coating offinely divided carbon (not 6 shown) may be interposed between thephotoconductive and electroluminescent layers. Such a carbon layer maybe formed to provide longitudinal conduction between the two layerswhile being substantially non-conducting along its surface dimension.Alternatively, undesirable regeneration may be inhibited by a judiciouschoice of the photoconductive and electroluminescent materials such thatthe wavelength response characteristics of the photoconductive andelectroluminescent layers do not overlap.

Another embodiment, shown in FIGURE 5, may be used as a detector ofatomic radiation. This embodiment comprises the same transducerdisclosed in conjunction with FIGURE 1, with the additional feature ofan added layer 22 of a substance which is responsive to atomicradiation. Element 22 may comprise a layer of chemical compoundresponsive to the radiation of alpha, beta, or gamma radiation, such asnaphthalene or anthnacene, enclosed in a glass structure 23 affixed toelectrode 13. Upon bombardment by atomic articles or rays, theradiation-sensitive layer 22 emits radiations which in turn aifect thedielectric constant of the photoconductive layer in the manner describedin connection with the embodiments of FIGURES 1 and 4. As previouslyexplained, such changes in the dielectric constant of thephotoconductive layer result in corresponding changes in the voltageimpressed across the electroluminescent layer with a resulting emissionof visible light radiation which may be observed through electrode 16.

Additional embodiments within the scope of the present invention includecascade structures in which additional photoconductive andelectroluminescent layers are arranged so that the light output from onestage may be directed to a second stage to provide further imageintensification.

The present invention provides a compact and simple structure foramplifying light radiations which is particularly useful in theconstruction of an efiicient and commercially practicable projectiontelevision system. It also provides an image converter for thetranslation of light radiations of one wavelength into those of anotherby the use of a compact and simple structure which is well adapted toeconomical fabrication on a mass production basis. The invention is alsoadaptable to other types of radiant-energy transducers, such as atomicradiation detectors and the like.

While particular embodiments of the invention have been shown anddescribed, modifications may be made and it is intended in the appendedclaims to cover all such modifications as fall Within the true spiritand scope of the invention.

1 claim:

1. A radiant-energy transducer comprising: a first electrode; aphotoconductive element in registration with said electrode and havingan electrical impedance per unit area which is variable in response toincident radiations; an electroluminescent element, responsive to anapplied electrical potential exceeding a predetermined thresholdmagnitude 'for emitting light radiations, in registration with saidphotoconductive element; a second electrode in juxtaposition with saidelectroluminescent elernent opposite said photoconductive element; saidfirst electrode being at least substantially transparent to saidincident radiations and said second electrode being at leastsubstantially transparent to said light radiations; means for projectingincident radiations through said first electrode upon saidphotoconducti-ve element to cause variations in its electrical impedanceper unit area; and means, including a source of unidirectional potentialsuperimposed upon an alternating potential with said unidirectional andalternating potentials individually of less magnitude than saidthreshold potential magnitude but together having a magnitude exceedingsaid threshold potential magnitude, for establishing an electricalpotential diiference between said electrodes to cause saidelectroluminescent element to emit light in response to the variationsin the voltage 7 applied to said electroluminescent element resultingfrom said variations in said electrical impedance of saidphotoconductive element.

2. A radiant-energy, transducer comprising: a first electrode; apho-toconductive element in registration with said electrode and havingan electrical impedance per unit area which is variable in response toincident radiation; an electroluminescent element, responsive to anapplied electrical potential exceeding a predetermined thresholdmagnitude for emitting light radiation, in registration with saidphotocondu'ctive element; a second electrode in juxtaposition with saidelectroluminescent element opposite said photoconductive element; saidfirst electrode being at least substantially transparent to saidincident radiation and said second electrode being at leastsubstantially transparent to said light radiation; means for projectingincident radiation through said first electrode upon saidphotoconductive element to cause variations in its electrical impedanceper unit area; and means, including a source of unidirectional potentialsuperimposed upon an alternating potential, for establishing anelectrical potential difference between said electrodes to cause saidelectroluminescent element to emit light in response to the variationsin the voltage applied to said electroluminescent element resulting fromsaid variation in said electrical impedance of said photoconductiveelement.

References Cited in the file of this patent UNITED STATES PATENTS2,120,916 Bitner June 14, 1938 2,645,721 Williams July 14, 19532,650,310 White Aug. 25, 1953 OTHER REFERENCES Thornton: A.C.D.C.Elec-troluminescence; Physical Review; vol. 113; No. 5; Mar. 1, 1959;pp. 1187-1191.

1. A RADIANT-ENERGY TRANSDUCER COMPRISING: A FIRST ELECTRODE; APHOTOCONDUCTIVE ELEMENT IN REGISTRATION WITH SAID ELECTRODE AND HAVINGAN ELECTRICAL IMPEDANCE PER UNIT AREA WHICH IS VARIABLE IN RESPONSE TOINCIDENT RADIATIONS; AN ELECTROLUMINESCENT ELEMENT, RESPONSIVE TO ANAPPLIED ELECTRICAL POTENTIAL EXCEEDING A PREDETERMINED THRESHOLDMAGNITUDE FOR EMITTING LIGHT RADIATIONS, IN REGISTRATION WITH SAIDPHOTOCONDUCTIVE ELEMENT; A SECOND ELECTRODE IN JUXTAPOSITION WITH SAIDELECTROLUMINESCENT ELEMENT OPPOSITE SAID PHOTOCONDUCTIVE ELEMENT; SAIDFIRST ELECTRODE BEING AT LEAST SUBSTANTIALLY TRANSPARENT TO SAIDINCIDENT RADIATIONS AND SAID SECOND ELECTRODE BEING AT LEASTSUBSTANTIALLY TRANSPARENT TO SAID LIGHT RADIATIONS; MEANS FOR PROJECTINGINCIDENT RADIATIONS THROUGH SAID FIRST ELECTRODE UPON SAIDPHOTOCONDUCTIVE ELEMENT TO CAUSE VARIATIONS IN ITS ELECTRICAL IMPEDANCEPER UNIT AREA; AND MEANS, INCLUDING A SOURCE OF UNIDIRECTIONAL POTENTIALSUPERIMPOSED UPON AN ALTERNATING POTENTIAL WITH SAID UNIDIRECTIONAL ANDALTERNATING POTENTIALS INDIVIDUALLY OF LESS MAGNITUDE THAN SAIDTHRESHOLD POTENTIAL MAGNITUDE BUT TOGETHER HAVING A MAGNITUDE EXCEEDINGSAID THRESHOLD POTENTIAL MAGNITUDE, FOR ESTABLISHING AN ELECTRICALPOTENTIAL DIFFERENCE BETWEEN SAID ELECTRODES TO CAUSE SAIDELECTROLUMINESCENT ELEMENT TO EMIT LIGHT IN RESPONSE TO THE VARIATIONSIN THE VOLTAGE APPLIED TO SAID ELECTROLUMINESCENT ELEMENT RESULTING FROMSAID VARIATIONS IN SAID ELECTRICAL IMPEDANCE OF SAID PHOTOCONDUCTIVEELEMENT.